The need for optical quality single crystals of materials exhibiting nonlinear optical properties is well established in the art. Potassium titanyl phosphate (i.e. KTP) is particularly useful in nonlinear optical devices, as described, for example, in U.S. Pat. No. 3,949,323. Optical quality crystals having dimensions on the order of one millimeter or more are particularly useful for many optical applications.
Processes for producing optical quality crystals using aqueous systems are known generally in the art as hydrothermal processes, and processes for producing such crystals using nonaqueous molten salt systems are known generally in the art as flux processes. Hydrothermal processes are considered particularly advantageous relative to flux processes for growing crystals for certain applications since hydrothermal processes often produce crystals having better optical damage resistance and relatively lower ionic conductivity. Typically, hydrothermal processes involve growing the crystal in a vessel having a growth region where the crystal grows and a nutrient region containing nutrient for growing the crystal, and employ an aqueous mineralizer solution (i.e., a mineralizer).
For example, one type of mineralizer used for hydrothermal crystal growth of KTP uses a mineralizer containing both potassium and phosphate. A typical commercial production of KTP crystals, described by Belt et al., SPIE Proceedings, 968, 100 (1988), uses a potassium phosphate mineralizer at a temperature in the range of about 520.degree. C. to 560.degree. C. and a pressure in the range of about 1700 to 2000 atmospheres. The relatively high temperature and pressure employed in this process makes scale-up difficult and expensive. As suggested in Laudise et al., "Solubility and P-V-T Relations and the Growth of Potassium Titanyl Phosphate", Journal of Crystal Growth, 102, pp. 427-433 (1990) the use of more moderate conditions can lead to a problem of the coprecipitation of anatase (TiO.sub.2), making the process less useful. Mineralizers rich in potassium have been used under conditions of fairly moderate temperature (e.g., 275.degree. to 425.degree. C.) and pressure (e.g., less than 14,000 psi) as described in U.S. Pat. No. 5,066,356. Although U.S. Pat. No. 5,066,356 demonstrates that growth temperature and pressure can be reduced while using certain concentrated mineralizer solutions, the growth rate of that process is somewhat limited due to the relatively low solubility of KTiOPO.sub.4 in the mineralizer. U.S. Pat. No. 4,305,778 discloses a hydrothermal process for crystal growth which utilizes a stable glass composition that minimizes the tendency of the seed crystals to dissolve in the aqueous mineralizer solution before nutrient can migrate to the seed crystals.
Another type of mineralizer used for the hydrothermal growth of KTiOPO.sub.4 involves the use of potassium fluoride solutions as described, for example, in Jia et al., "The Solubility of KTiOPO.sub.4 (KTP) in KF Aqueous Solution Under High Temperature and High Pressure", Journal of Crystal Growth, 79 (1986), pp. 970-973, and in Jia et al., "Hydrothermal Growth of KTP Crystals in the Medium Range of Temperature and Pressure", Journal of Crystal Growth, 99 (1990), pp. 900-904. Jia et al. disclose that by utilizing KF as a mineralizer relatively lower temperature and pressure can be employed for a hydrothermal KTP crystal growth process, and use of a pressure as low as 1000 Kg/cm.sup.2 (i.e. 14223 psi) is exemplified. The use of pure KF mineralizer as described by Jia et al. provides a solubility of about 2% under the stated growth conditions, but like the process described in Laudise et al. supra, the higher solubility occurs near the phase stability boundary (with respect to temperature, pressure and mineralizer concentration), so the possibility of coprecipitation of an undesirable non-KTP phase exists.
A generally recognized family of KTP-type materials has the formula MTiOXO.sub.4 where M is selected from the group consisting of NH.sub.4, K, Rb, Tl, mixtures thereof and mixtures of Cs therewith, and X is selected from the group consisting of P, As and mixtures thereof. Hydrothermal processes for producing KTiOPO.sub.4 are well studied. A problem associated with extending these studies to hydrothermal production of other crystals of the family involves the compositional variability of the mineralizers most suitable for growing each of the MTiOXO.sub.4 family of materials. These variations make the development of new crystals, such as KTiOAsO.sub.4, difficult and costly, since a new set of desirable growth parameters must be found for each of the isomorphs and their solid solutions.